The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Operating an internal combustion engine under lean of stoichiometry conditions can improve fuel efficiency, but may result in higher oxides of nitrogen (‘NOx’) emissions. Known aftertreatment systems for internal combustion engines operating lean of stoichiometry can include a three-way catalytic converter followed by a lean-NOx reduction catalyst, also referred to as a lean NOx trap (‘LNT device’), which can be used in concert with other exhaust aftertreatment devices, e.g., a selective catalytic reduction device (‘SCR device’). Known three-way catalytic converters (‘TWC’) function to reduce engine-out hydrocarbon (‘HC’), carbon monoxide (‘CO’), and NOx emissions during stoichiometric engine operation and HC and CO emissions during rich operation.
The SCR device includes a catalyst that promotes the reaction of NOx with a reductant, such as ammonia or urea, to produce nitrogen and water. The reductants may be injected into an exhaust gas feedstream upstream of the SCR device, requiring an injection system, a reductant tank and a control scheme. The tank may require periodic refilling and can freeze in cold climates requiring additional heaters and insulation. Additionally, engine operation may use the TWC to produce ammonia for use as a reductant.
Catalysts used in SCR devices have included vanadium (V) and tungsten (W) on titanium (Ti). Recently, mobile applications have begun adapting base metals including iron (Fe) or copper (Cu) with a zeolite washcoat. Copper catalysts may perform effectively at lower temperatures but have poor thermal durability. Iron catalysts may perform well at higher temperatures but with decreasing reductant storage efficiency.
For mobile applications, SCR devices can have a preferred operating temperature range of 200° C. to 600° C. The temperature range may vary depending on the catalyst. This temperature range can decrease during or after higher load operations. Temperatures greater than 600° C. may cause reductants to breakthrough and degrade the SCR catalysts, while the effectiveness of NOx processing decreases at temperatures lower than 200° C.
The LNT device adsorbs NOx emissions during lean engine operation and operates most effectively within a temperature range between 250° C. to 450° C. with effectiveness decreasing above and below that temperature range. The LNT device reduces the adsorbed NOx emissions only above a light-off temperature. Therefore it is advantageous to maintain the LNT device and SCR device above the light-off temperatures.